1. Field of the Invention
The present invention relates to a torque limiter, and in particular to a reel stand used in recording and reproduction apparatuses such as, for example, video tape recorders and audio tape recorders, and acting as a torque limiter.
2. Description of the Related Art
A conventional reel stand acting as a torque limiter will be described.
FIG. 5 is a cross-sectional view of a conventional reel stand 500 acting as a torque limiter, which is described in, for example, Japanese Laid-Open Publication No. 11-273188.
The reel stand 500 includes an upper reel stand section 52 and a lower reel stand section 53. A magnetic tape 51 is wound around a reel 50 built in a cassette (not shown).
The upper reel stand section 52 on which the reel 50 is to be placed is rotatably outserted into a rotation shaft 1. A disc-shaped yoke plate 54 is formed of a ferromagnetic material such as, for example, rolled steel and is integrally formed with the upper reel stand section 52. The upper reel stand section 52 and the yoke plate 54 may be outsert-molded. A magnet 55 is adsorbed to the yoke plate 54 by a magnetic force generated by the magnet 55, and is rotated with the yoke plate 54. A magnetic flux (not shown) generated by the magnet 55 forms a magnetic loop which passes through the yoke plate 54.
The lower reel stand section 53 is rotatably outserted into the rotatable shaft 1, like the upper reel stand section 52. The lower reel stand section 53 is integrally formed with a disc-shaped hysteresis plate 56. The hysteresis plate 56 is formed of a material having a magnetic hysteresis loss. The lower reel stand section 53 and the hysteresis plate 56 may be outsert-molded. A gear section 12 engageable with an external gear (not shown) is provided around an outer circumference of the lower reel stand section 53. The lower reel stand section 53 is attracted toward the upper reel stand section 52 by the magnetic force generated by the magnet 55, and a washer 10 included in the lower reel stand section 53 is in contact with a contact portion 57 of the upper reel stand section 52 in a thrust direction. The washer 10 is often coated with oil (not shown) for the purpose of stabilizing a clutch torque generated in the reel stand 500. The rotation shaft 1 is also often coated with oil for the purpose of stabilizing the rotation of the entirety of the reel stand 500.
Hereinafter, an operation of the reel stand 500 will be described.
By the magnetic force generated by the magnet 55, a friction torque is generated between the washer 10 included in the lower reel stand section 53 and the contact portion 57. When the external gear (not shown) drives and rotates the lower reel stand section 53, the hysteresis plate 56 crosses the magnetic loop generated by the magnet 55, thus generating a magnetic hysteresis torque. The friction torque and the hysteresis torque act together as the clutch torque generated between the upper reel stand section 52 and the lower reel stand section 53. The reel stand 500, which is rotated by the external gear, rotates the reel 50 while supplying an appropriate tension to the magnetic tape 51 by the clutch torque.
With the conventional reel stand 500, it is difficult to stabilize the clutch torque. The clutch torque generated by the reel stand 500 needs to be constantly stable without being influenced by various external disturbances. More specifically, the clutch torque needs to have a stable value regardless of the rotation rate of the reel stand 500, environmental temperature and humidity of use, the length of time that the reel stand 500 has been used, and the like. The value of the friction torque, which is one component of the clutch torque, varies in accordance with the friction coefficient between the washer 10 and the contact portion 57 and the inner and outer diameters of the contact portion 57. The value of the friction coefficient is significantly influenced by the amount oil coating the washer 10 and how the washer 10 is coated with oil (e.g., whether the washer 10 is coated with oil uniformly or non-uniformly). The value of the friction coefficient varies in accordance with the rotation rate of the reel stand 500, environmental temperature and humidity of use, the length of time that the reel stand 500 has been used, and the like. One measure for stabilizing the clutch torque is to reduce the friction torque, which is difficult with the conventional reel stand 500. Since a wall 11 exists between the rotation shaft 1 and the contact portion 57, it is impossible to reduce the inner and outer diameters of the contact portion 57. Therefore, the friction torque cannot be reduced. There are the following additional problems. (1) In the case where the washer 10 is not sufficiently coated with oil, the washer 10 occasionally becomes out of oil while the reel stand 500 is being used. (2) The abrasion particles of the washer 10 generated by the long-time use enter between the washer 10 and the contact portion 57, generating abnormal abrasion of the washer 10.